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Oil
Field Sump Tank Explosion
An abrupt, catastrophic rupture of a sump tank
occurred. The sump tank was actually not a "normal"
sump tank, but was a two compartment tank. A solid,
full section, plate of steel, separated both tank
sections and there was no passageway for gases or
fluids to pass from one compartment to the other.
The subject sump tank was partially buried. The two
tanks were welded together, giving a visual
appearance of being a single tank. Operating
personnel would have had absolutely no indication
that the subject tank was, in fact, two completely
independent tanks. The tank has been described as a
short tank (which contained sump liquid) and a long
tank (which contained a non-functional pressure
gauge). The two tanks were separated by a solid
baffle which prevented gaseous or liquid
communication between the long and short tank
sections.
Evidently, the workmen were attempting to
pressurize the larger tank section in order to
"blowout" or displace sump tank fluids from the
smaller tank section into a nearby waste storage
vessel.
It was reported that the pressure source being used
was gas from the high pressure gas sales line. It
was further reported that the gas pressure being
used by the workman at the time of the rupture was
probably about 800 psi. The failed long tank,
sitting on a two wheel dolly, is shown in Photograph
A.
Photograph A: Overall profile of the long tank. The failed circumferential weld is located at the right end of the tank as presented in this photograph (see white arrows).
The circumferential weld which failed is shown to
the right in Photograph A (see white arrows). A
"rear" quarter view, showing an overall view of the
failed end of the long tank is shown in Photograph
B.
Photograph B: Rear quarter view of the long tank, showing the failed circumferential weld (see red arrows).
A side profile view of the "short" tank is shown in
Photograph C.
Photograph C: Profile view of the short tank. The circumferential weld which failed (and which attached the long tank to the short tank) is shown at the left in this photograph (see red arrows).
The circumferential weld which failed is at the left
end of the short tank in Photograph C (see red
arrows). Photograph D shows the baffle plate which
completely separated the "short" tank from the
"long" tank.
Photograph D: Overall view of the failed weld and baffle plate on the short tank.
It is noted that the baffle shown in Photograph D is
slightly dished, i.e., concaved. It is evident that this "baffle" did
not allow liquid or gaseous communication between
the two tanks.
The long tank fracture surface and the mating short
tank fracture surface were photographed in detail.
Visual examination of the girth weld fracture
surface revealed three significant fracture
features. An overall view of the short tank fracture
surface is shown in Photograph E.
Photograph E: Overall view of a section of the failed circumferential weld on the short tank.
Red arrows denote location of welding slag in root
of circumferential weld.
The root of the circumferential weld contains what
appears to be slag and evidence of incomplete weld
root penetration. The slag is the darker, gray
material at the weld root located at the junction
between the weld metal and the outside diameter of
the baffle (see red arrows). Additionally, above the
incomplete root penetration area (toward the outside
weld surface), a darker discolored thickness of weld
metal is observed. This appears to be a region where
the weld failed progressively at some prior time,
under the influence of cyclic pressure, by metal
fatigue. The fact that this area of the fracture is
darker colored (more corroded), is evidence of crack
growth occurring over a period of time, as a result
of periodic or cyclic pressure.
A third area of fracture, created during the tank
rupture is located near the outside edge of the
failed weld (see white arrows). A close-up view of
the three distinct areas within the short tank
fracture is shown in Photograph F. The area
exhibiting lack of root penetration is denoted with
red arrows.
Photograph F: Close-up view of details of circumferential weld fracture. Red arrows denote slag and lack of weld rood penetration. The white bracket denotes an area
of old crack growth. The out (lighter colored), more recent fracture is denoted with white arrows.
The area of the fracture exhibiting "old fatigue
fracture" is denoted with a white bracket in
Photograph F. The area
of newer, fresher fracture is denoted with white
arrows.
When critical pressure levels occurred, the crack
increased in size. As a result, the thickness of
weld metal available to resist these pressurization
stresses diminished.
On the day of the incident, the pressure which
workmen put into the long tank was of sufficient
magnitude that the effective wall thickness that
remained in the girth weld was insufficient to
resist the resulting longitudinal stress and the vessel
rupture occurred.
As a result of the studies conducted in this matter,
it was determined that:
- Failure of the subject sump tank was the result of pressurization of the long tank.
- The girth or circumferential weld connecting the long and short tanks had been weakened by fatigue cracking and/or cyclic tearing resulting from prior pressurization during the vessel's lifetime.
- The subject long/short tank girth weld exhibited lack of weld root penetration. This lack of weld root penetration provided a possible source for metal fatigue and/or cyclic tearing crack initiation.
- The vessel, as configured at the time of the accident, did not have pressure relief valve. Such a relief valve would have limited the user's ability to apply pressures sufficient to result in crack growth within the subject girth weld. Such a relief valve would have reduced or eliminated the possibility of catastrophic vessel rupture.
- The pressure gauge on the subject tank was visually inoperable and thus could not be relied upon to indicate the tank internal pressure.
- The tank owner had not verified that the subject tank was suitable for use as a sump tank, i.e., by pressurizing one side of the tank, sump fluids
should be forced from the other side. That is, the tank owner had not verified that pressure communication
existed between the long and short tanks.
- The tank owner had not verified the integrity of the sump tank for its intended purpose. The tank owner had apparently not determined the integrity of the circumferential girth weld.
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Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- 